1. Field of the Invention
The present invention generally relates to communication systems and in particular to wireless communication systems.
2. Description of the Related Art
Communication systems have various communication channels some of which are used to convey (i.e., transmit and/or receive) information between users of the system and some of which are used to convey signaling information between various equipment of the communication system. The signaling information is also conveyed between system equipment and user equipment of the communication system. System equipment are equipment that are owned, operated and controlled by a system provider. Examples of system providers include local telephone companies, long distance companies, and Internet Service Providers. The system provider operates the communication system to provide communication services to the users. The signaling information is used by the system to manage the communication channels of the communication system. In particular, the manner in which communication is initiated, managed and terminated between users of the communication system is done with the use of signaling information.
In wireless communication systems such as systems that comply with the Universal Mobile Telecommunication System (UMTS) standard, system equipment such as base stations transmit signaling information to a user equipment (also called a mobile) to properly manage the user equipment and the communication channels being used by the user equipment. The signaling information that is transmitted from a base station to a mobile is called control information. The control information in UMTS HSDPA (High Speed Downlink Packet Access) standards (currently being developed) is transmitted over a shared channel because all the mobiles of the communication system receive their control information over this channel. The shared channel is called a High Speed Shared Control Channel (HS-SCCH). A user ID is appended to the control information to allow the control information to be processed by the intended mobile. An error detection code such as a Cyclic Redundancy Check (CRC) code is also appended to the control information. The CRC code allows a user for whom the control information is intended to determine whether the received control information contains errors. A user equipment receives the control information, decodes the received control information and if it is the intended user it begins to decode the data traffic channel HSDSCH (High Speed Downlink Shared Channel) scheduled for this user. Even if the traffic channel decode is unsuccessful, the partially decoded traffic information is stored in a buffer for combining with future re-transmissions in accordance with a Hybrid ARQ (Automatic reQuest) protocol. It is the information in the traffic Hybrid ARQ buffer that the user equipment uses to allow the system to efficiently manage the communication channels assigned to the user equipment. Therefore, it is critical that the user's traffic Hybrid ARQ buffer contain correct information and is not corrupted by erroneous information that would propagate along with future re-transmissions.
A user equipment (e.g., cellular phone, wireless personal computer, pager) thus needs to know if the received control information contains errors and furthermore needs to know if the control information is intended for the user equipment. When a user equipment attempts to decode traffic information intended for another user, the resulting decode information is corrupt leading to a false alarm. A false alarm is the decoding of received traffic information by a user where such information is not intended for the user. Thus, a false alarm situation also corrupts the user equipment's Hybrid ARQ buffer. Whether a given user is the intended recipient or not of the traffic is determined by the User Equipment with the aid of decoding a control channel (HS-SCCH) that has User ID information embedded in it. Of course, the User ID assigned by the system is a priori communicated to the User Equipment via upper layer messages at the time of call setup. Control channels have the User ID so embedded in them that only the intended user will normally be able to successfully decode the control channel; in other words a CRC check on the control information passes. If the CRC check fails then the user equipment decides that (a) the control information and the accompanying data traffic information was not intended for it or (b) even if the information were intended for the user equipment, the control information was adversely affected by channel errors and was invalid. In either case, the User Equipment discards the control information and does not attempt to decode the corresponding data traffic channel to avoid Hybrid ARQ buffer corruption. However, it may happen due to channel induced bit errors that the decode of the control channel information passes a CRC check for some user despite the transmitted (embedded) User ID not being the same as this user's ID. A false alarm event thus occurs when a user decodes a control channel in a manner where the CRC fails to detect the presence of errors in the decoded block and the user accepts the wrong information as valid information.
The control information contains I bits of information, the CRC information contains N bits of information and the user ID contains K bits of information. Correct user ID information will prevent false alarms. However, even if the user ID is correct, the user's control buffer can still be corrupted if the received control information contains errors. Consequently, it is prudent for the user equipment to determine whether both the user ID and the control information contain errors. There are two techniques that are used to detect errors in the control information and the user ID. These techniques tend to reduce the likelihood of the occurrence of false alarms.
A first technique is for the I-bit control information block to be appended to the K user ID bits and the resulting I+K bits are used to generate a CRC code of N bits which are appended to the I+K bits. The total I+K+N bits are then transmitted over the shared channel. A second technique is to first generate N bits of CRC code from the I-bit control information block and the K bits of user ID are converted to N bits (assuming K<N). The conversion of the K bits to N bits is done by zero padding, i.e., adding 0 bits to the K bits so that the total number of user ID bits is equal to N. The now N user ID bits are then modulo 2 added to the N CRC bits resulting in N user specific coded CRC bits. The N user specific coded CRC bits are appended to the I-bit control information block and the I+N bits are transmitted over the HS-SCCH. The second technique is more desirable since a fewer number of bits are transmitted over the HS-SCCH meaning less overhead in the transmission of signaling information. When K=N, there is no need to convert the K user ID bits, the K user ID bits are modulo 2 added to the N CRC bits resulting in N coded bits which are appended to the I control information bits which are then transmitted over the HS-SCCH. The probability of false alarms is thus set and is based on the value of N. To change or reduce the probability of false alarm N has to be changed. Because N is usually a set value, the probability of false alarms is usually set. When K<N, the probability of false alarms depends on how the conversion is done. What is therefore needed is a technique for converting the K user ID bits to N coded user ID bits where N is the number of bits in the CRC code generated from I control information bits and where K<N.